In vitro particles of Rous sarcoma virus (RSV) were assembled with Gag protein expressed in E. coli and E. coli RNA and examined in the STEM. They appeared very much like immature retroviruses and had ma ses of about 80 Md, corresponding to about 1200 Gag proteins per particle. Other constructions are being studied. In addition, several laboratories are becoming interested in measuring the masses of retroviruses since most data bn them is about 20 years old. Some preliminary results for RSV have been obtained. Two strains of (whole) in vivo virus particles were purified from infected turkey cells. One has a genorne of only Gag, Pol, and Env. The other carries an additional gene (alkaline phosphatase) making the genome -20% larger (like the "real" RSV which carries the src oncogene). The size of retroviruses appears variable and one hypothesis is that the size is determined by the amount of RNA packaged. Mass measurements from the STEM data indicate that the MWs appear about the same.